Catalyst for Use in the Selective Catalytic Reduction (SCR) of Nitrogen Oxides

ABSTRACT

The present invention pertains to a catalyst for use in the selective catalytic reduction (SCR) of nitrogen oxides comprising a monolithic substrate and a coating A, which comprises an oxidic metal carrier comprising an oxide of titanium and a catalytic metal oxide which comprises an oxide of vanadium wherein the mass ratio vanadium/titanium is 0.07 to 0.26.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. patent application Ser. No.17/268,852, which is a national phase entry of PCT/EP2019/072706, filedAug. 26, 2019, which is herein incorporated by reference in itsentirety, which claims priority to EP 18191131.4, filed Aug. 28, 2018,and EP 19171394.0, filed Apr. 26, 2019.

FIELD OF THE INVENTION

The present invention relates to a catalyst for use in the selectivecatalytic reduction (SCR) of nitrogen oxides by reaction with ammonia.It in addition relates to a method for the preparation of such catalyst,which is in particular based on the use of a washcoat slurry comprisingone or more catalyst metal precursor compounds dispersed onnanoparticles of an oxidic metal carrier.

BACKGROUND OF THE INVENTION

Catalysts, in particular if applied to monolithic catalyst supports, areuseful in treatment of gases by heterogeneous catalysis and are widelyused in automotive and stationary emission control devices and otherkind of reactors in the chemical industry.

Monolithic catalyst supports are extruded substrates or are made ofceramic materials, metals or of corrugated ceramic paper stacked up orrolled up a monolithic structured substrate with a plurality of parallelgas flow channels separated by thin walls that are coated with acatalytic active substance.

Typically, the catalytic active material is supported on the substrateby applying a washcoat, typically a refractory oxide layer deposited onthe catalyst substrate. The washcoat provides a porous, high surfacearea layer bonded to the surface of the substrate after drying, andoptionally calcining in a controlled atmosphere.

The washcoat is subsequently impregnated with a solution of thecatalytic active material or precursors thereof. Finally, the thuscatalyzed substrate is activated by calcination at elevatedtemperatures.

An important field of use of monolithic catalysts is the removal ofnitrogen oxides contained in off-gas.

When operating monolithic SCR catalyst in the removal of nitrogen oxides(NOx), the nitrogen oxides are converted to free nitrogen with areducing agent usually ammonia in the presence of the catalyst supportedon or within pores in the walls of the monolithic substrate by selectivecatalytic reduction according to reactions I and II:

4NO+4NH₃+O₂→4N₂+6H₂O  I

NO+NO₂+2NH₃→2N₂+3H₂O  II

Catalyst compounds active in the SCR reaction with ammonia are per seknown in the art. To name a few, typically employed catalysts arevanadium oxide, tungsten oxide and zeolitic materials, usually exchangedwith for example copper and/or iron, either used alone or as mixturesthereof. The most commonly used catalyst for the reduction of NOx fromoff-gas is titania-supported vanadium oxide, optionally promoted withtungsten oxide.

A problem with V₂O₅/TiO₂ and V₂O₅—WO₃/TiO₂ catalysts is that thesecatalysts are not sufficiently efficent in the ammonia SCR reaction atgas temperatures below 250° C.

A number of applications, however, require high SCR catalytic activityat gas temperatures as low as 100° C. For example stationary combustionfacilities, such as steam boilers, operate typically at with off-gastemperatures between 120 to 150° C.

For other scenarious, e.g. power plants, a heat recovery steam generator(HRSG) that recovers heat from a hot with off-gas stream may beinstalled. The HRSG unit comprises a superheater, an evaporator and aneconomizer.

In the superheater and evaporator, the heat in the with off-gas is usedto superheat steam and to preheat feed water before it is pumped to theboiler, which increases the boiler efficiency of the power plant. Thewith off-gas temperature is thereby typically cooled to approximately150° C. The low temperature downstream of the evaporator encounters aproblem in the NOx removal by means of SCR.

WO2016/058713 A1 discloses a method for preparing a catalyzed fabricfilter which comprises impregnating the fabric filter substrate with anaqueous impregnation liquid comprising an aqueous hydrosol of one ormore catalyst metal precursor compounds dispersed on nanoparticles of anoxidic metal carrier.

According to the disclosure of WO 2016/058713 A1 (see method ofExample 1) a NH₃VO₃/TiO₂ ratio of 0.58 is used which corresponds to aV/Ti ratio of 0.42.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the NO conversion activity of ammoniumvanadate/titania preparations.

FIG. 2 is a graph showing the particle size distribution of ammoniumvanadate/titania particles.

DETAILED DESCRIPTION

The inventors of the present invention now have surprisingly found thatthis ratio can be decreased, i.e. less vanadium can be used, withouteffecting the DeNOx activity of the catalyst. At the same time, SO₂oxidation is reduced which subsequently will lead to less formation ofammonium bisulphate (ABS).

Accordingly, the present invention pertains to a catalyst for use in theselective catalytic reduction (SCR) of nitrogen oxides comprising

-   -   a monolithic substrate and    -   a coating A which comprises an oxidic metal carrier comprising        an oxide of titanium and a catalytic metal oxide which comprises        an oxide of vanadium wherein the mass ratio vanadium/titanium is        0.07 to 0.26.

Preferred embodiments of the invention are disclosed and discussed inthe following. These embodiments can either be employed each alone or incombination thereof.

The monolithic substrate can be a honeycomb substrate made of ceramicmaterial, in particular cordierite, or metal.

However, a preferred monolithic substrate contains glass fibers. Thesesubstrates have shown to have no inadequate influence on the catalyticactivity of the final catalyst.

A monolithic substrate containing glass fibers preferably comprisescorrugated sheets of glass fiber. It is for example produced bycorrugating flat sheets of a glass fiber web and stacking a plurality ofthe corrugated sheets to a monolithic structure or rolling-up a singlecorrugated sheet to a cylindrically monolith.

Preferably, each of the corrugated sheets is provided with a flat linermade of the same material as the corrugated sheet(s), before stacking orrolling up.

The catalytic metal oxide of coating A which comprises an oxide ofvanadium preferably comprises vanadium pentoxide (V₂O₅). Besides itoptionally comprises oxides of other metals, in particular an oxide oftungsten and/or molybdenum, preferred tungsten trioxide (WO₃) and/ormolybdenum trioxide (MoO₃).

The oxidic metal carrier of coating A which comprises an oxide oftitanium usually comprises titanium dioxide. Besides it optionallycomprises an oxide of another metal, in particular an oxide of aluminum,cerium, zirconium or mixtures, mixed oxides or compounds comprising atleast one of these oxides.

Preferably, the oxidic metal carrier consists of either single oragglomerated nanoparticles of titanium dioxide with a primary particlesize of between 10 and 150 nm.

According to the present invention the mass ratio vanadium/titanium is0.07 to 0.22 and preferably 0.1 to 0.21. For the avoidance of doubt, themass ratio is calculated based on the mass of vanadium metal andtitanium metal.

Preferably, the inventive catalyst is free of platinum group metals, inparticular free of palladium.

In an embodiment of the present invention the inventive catalystcomprises an additional coating B directly on the monolithic substrateand below coating A. Coating B preferably comprises an oxide oftitanium, in particular titanium dioxide, and optionally of aluminum,cerium, zirconium or mixtures, mixed oxides or compounds comprising atleast one of these oxides.

In another embodiment of the present invention, the oxidic metal carriernot only comprises an oxide of titanium, in particular titanium dioxide,but in addition an oxide of vanadium, in particular vanadium pentoxideand optionally oxides of tungsten and/or molybdenum, in particulartungsten trioxide and/or molybdenum trioxide.

This is in particular the case if a recycled or fresh SCR catalystcomprising vanadium pentoxide (V₂O₅) and titanium dioxide TiO₂, andoptionally tungsten trioxide (WO₃) and/or molybdenum trioxide (MoO₃) isreused.

In still another embodiment, the monolithic substrate is an extrudatecomprising vanadium pentoxide (V₂O₅) and titanium dioxide (TiO₂) andoptionally tungsten trioxide (WO₃) and/or molybdenum trioxide (MoO₃).

Preferably, the inventive catalyst comprises from about 5 to about 95%by weight of the catalytically active material.

The present invention also pertains to a process for preparing acatalyst for use in the selective catalytic reduction (SCR) of nitrogenoxides comprising the steps of

a) providing a monolithic substrate

b) providing an aqueous washcoat slurry comprising one or more catalystmetal precursor compounds comprising a vanadium compound dispersed onparticles of an oxidic metal carrier comprising titanium oxide;

c) impregnating the monolithic substrate with the washcoat slurry; and

d) drying and thermally activating the impregnated monolithic substrateat a temperature 150 to 600° C. to convert the one or more metalprecursor compounds to their catalytically active form.

As regards step b) the catalyst metal precursor compound comprising avanadium compound is preferably ammonium metavanadate. In case theinventive catalyst comprises oxides of tungsten and/or molybdenum, thewashcoat slurry preferably comprises ammonium metatungstate and/orammonium heptamolybdate.

In some applications of the process according to the invention themonolithic substrate is pre-coated prior to the impregnation of themonolithic substrate to provide a larger surface area of the substrate.

Thus, in an embodiment of the present invention, the monolithicsubstrate is washcoated with an oxidic metal carrier comprising oxidesof titanium and optionally of aluminum, cerium, zirconium or mixtures,mixed oxides or compounds comprising at least one of these oxides, priorto impregnating the monolithic substrate with the washcoat slurry instep (c).

The thermally activation of the impregnated monolithic substrateaccording to step d) can be performed either before installation of theimpregnated substrate or after installation of impregnated substrate inan SCR unit. Preferably, the thermal activation temperature is between300-450° C.

In case the inventive process comprises providing a recycled catalyst instep a) which already comprises vanadium pentoxide (V₂O₅) and titaniumdioxide TiO₂, and optionally tungsten trioxide (WO₃) and/or molybdenumtrioxide (MoO₃), the amount of the precursor of the said oxides in theaqueous washcoat slurry can be reduced.

In the preparation of the aqueous washcoat slurry as disclosed in moredetail below, the oxidic metal carrier is present in the impregnationsolution in form of a hydrosol, a part of which can gelate duringstorage and agglomerate to a larger particle size than the preferredsize.

Primary amines have shown to prevent agglomeration or to break downalready formed agglomerates. It is thus preferred to add a dispersingagent to the washcoat slurry selected from one or more of primaryamines.

The primary amine is preferably soluble in the aqueous washcoat slurry,when having been added in an amount resulting in the above disclosedpurpose. Primary amines with fewer than seven carbon atoms are watersoluble, preferred primary amines for use in the invention are thereforemono-methyl amine, mono-ethyl amine, mono-propyl amine, mono-butyl amineor mixtures thereof. Of these, the most preferred dispersing agent ismono-ethyl amine.

Good results are obtained when the primary amine dispersing agent isadded to the washcoat slurry in an amount resulting in a pH value of thewashcoat slurry above 7.

As known in the art, an effective catalyst requires a monolayer coverageof the catalytically active material on an oxidic metal carrier.Formation of excess crystalline catalytically active material on thecarrier should be avoided.

A high dispersion of catalytically active material after thermalactivation at low temperatures is possible when employing the so-calledequilibrium deposition method in combination with the primary aminedispersing agent. In this method the electrostatic force of attractionbetween oppositely charged metal compounds is utilized to bind one metalcompound on surface of the oppositely charge metal compound in finelydispersed form. TiO₂ and V_(x)O_(y) ^(z−) have opposite surface chargesin the pH interval of 4 to 6. Hence, the electrostatic force ofattraction facilitates deposition of V_(x)O_(y) ^(z−) onto TiO₂.

The limited solubility of V_(x)O_(y) ^(z−) is circumvented by theprincipal of Le Chatelier, when V_(x)O_(y) ^(z−) binds to Ti—OH₂ ⁺sites, new V_(x)O_(y) ^(z−) ions are solubilized. This continuousprocess occurs at room temperature and requires nothing but stirring andpH regulation.

Thus, the aqueous washcoat slurry is preferably prepared by the steps of

i) adding the one or more catalyst metal precursor compounds and theoxidic metal carrier to water and continuously adding an acid to theliquid to maintain the pH of the liquid at a value where the surfacecharge of the one or more catalyst precursor metal compound is negativeand the Zeta potential of the oxidic metal carrier is positive;

ii) adsorbing the one or more catalyst metal precursor compound onto thesurface of the oxidic metal carrier; and optionally

iii) adding the dispersing agent to the thus prepared liquid in anamount to obtain a pH value above 7 of the thus prepared washcoatslurry.

Preferably, the pH value in step (i) and (ii) is maintained at between2.5 and 5.

The catalyst of the present invention is suitable for the treatment ofoff-gas from automotive and stationary sources and to selectively reducenitrogen oxides contained in said off-gas.

Accordingly, the present invention further relates to a method fortreating the off-gas from automotive and stationary sources and toselectively reduce nitrogen oxides contained in said off-gas,characterized in that the off-gas is passed over an inventive catalyst.

Example 1

a) Preparation of an ammonium vanadate/titania containing washcoatslurry

-   1. 2250 g of demineralized water is mixed with 2630 g TiO2.-   2. 381 g of ammonia meta vanadate (AMV) is added under continuous    stirring. The NH₄VO₃/TiO₂—ratio is 0.16.-   3. The pH is monitored and increases continuously.-   4. The pH is adjusted with concentrated nitric acid in the interval    of 4.0-4.5.-   5. After a few hours the pH of the liquid remains constant and the    slurry is left under constant stirring for at least 24 hours.    However, the pH needs adjustment every 3 hours.    -   The resulting liquid has a red color and 100 g ethylamine (70%        in water) is added (or until pH ˜9.2-9.5).    -   The liquid becomes ivory white and is subsequently milled down        to a particle size of 250-400 nm (D50).    -   Optionally a 5.6 g of surfactant is added to increase wash-coat        uptake on the corrugated substrate

b) A monolithic substrate consisting of glass fibers is washcoated withthe aqueous slurry. The V/Ti ratio of the catalyst thus obtained is0.12.

Example 2

The method of Example 1 was repeated with the exception that thecatalyst obtained has a V/Ti ratio of 0.21

Example 3

The method of Example 1 was repeated with the exception that thecatalyst obtained has a V/Ti ratio of 0.24

Example 4

The method of Example 1 was repeated with the exception that thecatalyst obtained has a V/Ti ratio of 0.25

Example 5

a) Preparation of an ammonium vanadate/titania containing washcoatslurry

-   1. 2250 g of demineralized water is mixed with 2630 g TiO2.-   2. 381 g of ammonia meta vanadate (AMV) is added under continuous    stirring. The NH₄VO₃/TiO₂—ratio is 0.16.-   3. The pH is monitored and increases continuously.-   4. The pH is adjusted with concentrated nitric acid in the interval    of 4.0-4.5.-   5. After a few hours the pH of the liquid remains constant and the    slurry is left under constant stirring for at least 24 hours.    However, the pH needs adjustment every 3 hours.    -   The resulting liquid has a yellow color and 100 g ethylamine        (70% in water) is added (or until pH ˜9.2-9.5).    -   The liquid becomes pale yellow and is subsequently milled down        to a particle size of 600-700 nm (D50).    -   Optionally a 5.6 g of surfactant is added to increase wash-coat        uptake on the corrugated substrate

b) A monolithic substrate consisting of glass fibers is washcoated withthe aqueous slurry. The V/Ti ratio of the catalyst thus obtained is0.12.

Tests

A part of each of the aqueous slurries obtained according to Examples 1to 4 was dried and the powders thus obtained were tested in the ammoniaSCR of NO with a gas composition containing 500 ppm NO, 533 ppm NH₃, 10vol % O₂, 4% H₂O and balance N₂. The results of the tests are summarizedin FIG. 1.

As is apparent from FIG. 1, the powders corresponding to the catalystsof Examples 1 to 4 according to the invention possess very good NOconversion activity at higher temperatures. It is also apparent thatincreasing the Ti/V ratio above a value of 0.20, in particular above0.24 does not result in a higher activity, the activity is levelled out.

On the other hand, it is the general knowledge of a person skilled inthe field of selective catalytic reduction of nitrogen oxides that the(unwanted) oxidation of SO₂ increases with an increasing Ti/V ratio(see, for example, Applied Catalysis b: Environmental 19 (1998)103-117).

Accordingly, the present invention allows to gain an optimal NOconversion while restricting the unwanted SO₂ oxidation to a minimum.

FIG. 2 shows the particle distribution of Example 5.

It has to be noted that the milling can also be performed after stepa)1. as described in Examples 1 and 2. This means that the slurry ofTiO₂ in demineralized water can be milled prior to the addition ofammonium metavanadate.

The skilled person knows how to adjust milling conditions in order toobtain a desired particle size. Suitable particle sizes of the one ormore catalyst metal precursor compounds comprising a vanadium compounddispersed on particles of an oxidic metal carrier comprising titaniumoxide (D50) are 200 to 750 nm, preferably 250 to 600 nm, more preferably300 to 500 nm.

As described above, the milling can take place prior to the addition ofammonium metavanadate, i.e. after step a)1., or after the adjustment ofthe pH with an amine, i.e. during step a)5. as described above inExamples 1 to 5.

Exemplary Embodiments

1: A catalyst for use in the selective catalytic reduction (SCR) ofnitrogen oxides comprising

-   -   a monolithic substrate and    -   a coating A which comprises an oxidic metal carrier comprising        an oxide of titanium and a catalytic metal oxide which comprises        an oxide of vanadium wherein the mass ratio vanadium/titanium is        0.07 to 0.26.

Embodiment 2: A catalyst according to Embodiment 1, wherein themonolithic substrate comprises corrugated sheets of glass fiber.

Embodiment 3: A catalyst according to Embodiment 1 and/or 2, wherein thecatalytic metal oxide of coating A which comprises an oxide of vanadiumcomprises vanadium pentoxide (V2O5).

Embodiment 4: A catalyst according to Embodiment 3, wherein thecatalytic metal oxide of coating A which comprises an oxide of vanadiumcomprises in addition an oxide of tungsten and/or molybdenum.

Embodiment 5: A catalyst according to one or more of Embodiments 1 to 4,wherein the oxidic metal carrier of coating A which comprises an oxideof titanium comprises titanium dioxide.

Embodiment 6: A catalyst according to Embodiment 5, wherein the oxidicmetal carrier of coating A which comprises an oxide of titaniumcomprises in addition an oxide of aluminum, cerium, zirconium ormixtures, mixed oxides or compounds comprising at least one of theseoxides.

Embodiment 7: A catalyst according to one or more of Embodiments 1 to 6,wherein the oxidic metal carrier consists of either single oragglomerated nanoparticles of titanium dioxide with a primary particlesize of between 10 and 150 nm.

Embodiment 8: A catalyst according to one or more of Embodiments 1 to 7,wherein the mass ratio vanadium/titanium is 0.1 to 0.21.

Embodiment 9: A catalyst according to one or more of Embodiments 1 to 8,which is free of platinum group metals.

Embodiment 10: A catalyst according to one or more of Embodiments 1 to9, which comprises an additional coating B directly on the monolithicsubstrate and below coating A.

Embodiment 11: A catalyst according to Embodiment 10, wherein coating Bcomprises an oxide of titanium and optionally of aluminum, cerium,zirconium or mixtures, mixed oxides or compounds comprising at least oneof these oxides.

Embodiment 12: A process for preparing a catalyst according to one ormore of Embodiments 1 to 11 comprising the steps of

a) providing a monolithic substrate

b) providing an aqueous washcoat slurry comprising one or more catalystmetal precursor compounds comprising a vanadium compound dispersed onparticles of an oxidic metal carrier comprising titanium oxide;

c) impregnating the monolithic substrate with the washcoat slurry; and

d) drying and thermally activating the impregnated monolithic substrateat a temperature 150 to 600° C. to convert the one or more metalprecursor compounds to their catalytically active form.

Embodiment 13: A process according to Embodiment 12, wherein thecatalyst metal precursor compound comprising a vanadium compound isammonium metavanadate.

Embodiment 14: A process according to Embodiment 12 and 13, wherein theaqueous washcoat slurry comprises a primary amine soluble in thatliquid.

Embodiment 15: A process according to Embodiment 14, wherein the primaryamine is mono-ethyl amine.

Embodiment 16: A process according to any one of Embodiments 12 to 15,wherein the one or more catalyst metal precursor compounds comprising avanadium compound dispersed on particles of an oxidic metal carriercomprising titanium oxide have a particle size D50 of 200 to 750 nm.

Embodiment 17: A method for treating the off-gas from automotive andstationary sources and selectively reduce nitrogen oxides contained insaid off-gas, wherein the off-gas is passed over a catalyst according toone or more of Embodiments 1 to 11.

1. A catalyst for use in the selective catalytic reduction (SCR) ofnitrogen oxides comprising a monolithic substrate and a coating A whichcomprises an oxidic metal carrier comprising an oxide of titanium and acatalytic metal oxide which comprises an oxide of vanadium wherein themass ratio vanadium/titanium is 0.07 to 0.22.
 2. The catalyst accordingto claim 1, wherein the mass ratio vanadium/titanium is 0.1 to 0.21. 3.The catalyst according to claim 1, wherein the monolithic substratecomprises wherein the monolithic substrate a) comprises corrugatedsheets of glass fiber, b) comprises corrugated ceramic paper stacked upor rolled up, c) comprises a corrugated sheets with a flat liner, or d)is an extrudate comprising vanadium pentoxide and titanium dioxide. 4.The catalyst according to claim 1, wherein the catalytic metal oxide ofcoating A which comprises an oxide of vanadium comprises vanadiumpentoxide (V₂O₅).
 5. The catalyst according to claim 4, wherein thecatalytic metal oxide of coating A which comprises an oxide of vanadiumcomprises in addition an oxide of tungsten and/or molybdenum.
 6. Thecatalyst according to claim 1, wherein the oxidic metal carrier ofcoating A which comprises an oxide of titanium comprises titaniumdioxide.
 7. The catalyst according to claim 6, wherein the oxidic metalcarrier of coating A which comprises an oxide of titanium comprises inaddition an oxide of aluminum, cerium, zirconium or mixtures, mixedoxides or compounds comprising at least one of these oxides.
 8. Thecatalyst according to claim 1, wherein the oxidic metal carrier consistsof either single or agglomerated nanoparticles of titanium dioxide witha primary particle size of between 10 and 150 nm.
 9. The catalystaccording to claim 1, which is free of platinum group metals.
 10. Thecatalyst according to claim 1, which comprises an additional coating Bdirectly on the monolithic substrate and below coating A.
 11. Thecatalyst according to claim 10, wherein coating B comprises an oxide oftitanium and optionally of aluminum, cerium, zirconium or mixtures,mixed oxides or compounds comprising at least one of these oxides.
 12. Aprocess for preparing a catalyst according to claim 1, comprising thesteps of a) providing a monolithic substrate b) providing an aqueouswashcoat slurry comprising one or more catalyst metal precursorcompounds comprising a vanadium compound dispersed on particles of anoxidic metal carrier comprising titanium oxide; c) impregnating themonolithic substrate with the washcoat slurry; and d) drying andthermally activating the impregnated monolithic substrate at atemperature 150 to 600° C. to convert the one or more metal precursorcompounds to their catalytically active form.
 13. The process forpreparing a catalyst according to claim 12, wherein the catalyst metalprecursor compound comprising a vanadium compound is ammoniummetavanadate.
 14. The process for preparing a catalyst according toclaim 12, wherein the aqueous washcoat slurry comprises a primary aminesoluble in that liquid.
 15. The process for preparing a catalystaccording to claim 14, wherein the primary amine is mono-ethyl amine.16. The process for preparing a catalyst according to claim 12, whereinthe one or more catalyst metal precursor compounds comprising a vanadiumcompound dispersed on particles of an oxidic metal carrier comprisingtitanium oxide have a particle size D50 of 200 to 750 nm.
 17. A methodfor treating the off-gas from automotive and stationary sources andselectively reduce nitrogen oxides contained in said off-gas, whereinthe off-gas is passed over a catalyst according to claim 1.